This invention relates generally to pixel array printing machines such as ink jet printers, and more particularly to a skewed substrate pixel array printing machine including a skewed printing area or skewed sheet supporting area, platen.
Liquid ink printers of the type frequently referred to either as continuous stream or as drop-on-demand, such as piezoelectric, acoustic, phase change wax-based or thermal, have at least one printhead from which droplets of ink are directed towards a recording sheet. Within the printhead, the ink is contained in a plurality of channels. For a drop-on-demand printhead power pulses cause the droplets of ink to be expelled as required from orifices or nozzles at the end of the channels.
In a thermal ink-jet printer, the power pulses are usually produced by formation and growth of vapor bubbles on heating elements or resistors, each located in a respective one of the channels, which are individually addressable to heat and vaporize ink in the channels. As voltage is applied across a selected resistor, a vapor bubble grows in the associated channel and initially expels the ink therein from the channel orifice, thereby forming a droplet moving in a direction away from the channel orifice and towards the recording medium where, upon hitting the recording medium, a dot or spot of ink is deposited. Following collapse of the vapor bubble the channel is refilled by capillary action, which, in turn, draws ink from a supply container of liquid ink. Operation of a thermal ink-jet printer is described in, for example, U.S. Pat. No. 4,849,774.
The ink jet printhead may be incorporated into either a carriage type printer, a partial width array type printer, or a page-width type printer. The carriage type printer typically has a relatively small printhead containing the ink channels and nozzles. The printhead can be sealingly attached to a disposable ink supply cartridge and the combined printhead and cartridge assembly is attached to a carriage which is reciprocated along a line normal to a supported recording medium to print one swath of information (equal to the length of a column of nozzles), at a time, on the supported, stationary recording medium, such as paper or a transparency.
After the swath is printed, the paper or the printhead is stepped a distance equal to the span of the printed swath or a portion thereof, so that the next printed swath is contiguous or overlapping therewith. This procedure is repeated until an entire page is printed. In contrast, the page width printer includes a stationary printhead that is mounted at right angle to the recording medium and has a length sufficient to print across the width or length of the supported recording medium at a time. The supported recording medium is continually moved past the page width printhead in a direction substantially normal to the printhead length and at a constant or varying speed during the printing process.
The trend in ink jet printer design is clearly towards higher and higher speed. Other than the number of nozzles in a printhead or the maximum firing frequency of the printhead, there are many other factors that affect the printing speed of an ink jet printer. Therefore, as print speeds are increased, the amount of printer time spent on activities, (such as properly registering an entire edge of a sheet to be printed on) other than actual printing can become a large portion of the time required by the printer for producing a page.
One of the well known designs for an ink jet printer involves transporting the printing sheet or paper on a rotatable drum platen. The advantages of printing on a drum platen, for example include inherent unidirectional printing, and the elimination of flyback time as is common in the case of an oscillating printhead mechanism on a flat non-rotating platen. The advantages also include a potential for proper image registration, since swath-to-swath or pass-to-pass sheet or paper advance errors are no longer an issue as in the case of an indexable sheet on a stationary flat platen. Typically, this architecture is used for single ejector or distributed ejectors where distances between the ejectors are many times the pixel size. In such a case the advance of the printhead is only one pixel for each rotation.
It has been found that when using an ink jet head or printhead as such, the disadvantages further include undesirably large accelerating and decelerating forces. Such forces are necessary for quickly and rapidly jump-moving, and stopping the printhead between passes or swaths, and in a registered position for printing the next swath. This of course is being done while the drum is rotating. One reason for the problem stems from the fact that the printhead must be advanced or jumped a significant fraction of its size or printing length between passes. Whereas the problem of large acceleration and deceleration forces can be avoided by using a continuously moving printhead, there are other problems including a "stair step" image defect, that are associated with printing as such in areas or substrates that are "squared to the platen", meaning areas or substrates in which the lead edge of the image is registered so that such edge is parallel to the axis of the rotating platen.
As the trend is toward larger printheads, increasing speed requires larger advances with similar required accuracies in the same amount of time. Furthermore, a gap must be left in the print region in order to accommodate this motion-degrading problem, therefore potentially eliminating any speed advantages of using a drum platen.